The present invention relates to an electrophotographic copying process for reversal development using a laminate-type electrophotographic photoreceptor containing a specific compound.
Electrophotography invented by C. F. Carlson is now used widely not only in the field of copying machines but also in the field of printers and facsimiles because of the capability of forming images showing excellent instantaneity, high-quality and retentivity.
This electrophotographic process is basically composed of an image forming process comprising the steps of uniformly charging the photoreceptor surface, forming a static latent image by image exposure corresponding to manuscript, developing the latent image with toner, transferring the toner image to a transfer paper (transfer may be conducted through an intermediate transfer) and fixing, and an initialization process for repeated use of the photoreceptor, that is, a charge erasure process comprising steps of cleaning for removing the residual developer and erasing residual electrical charges on the photoreceptor surface.
As the photoreceptor of electrophotography, there have conventionally been used inorganic photoconductive materials such as selenium, arsenic-selenium alloy, cadmium sulfide, zinc oxide and the like, but recently photoreceptors using organic photoconductive materials having advantages such as presenting no pollution problems, facilitativity of film formation, facilitativity of production of the photoconductor, etc., have been developed.
Particularly, laminate-type photoreceptors having a charge generation layer and a charge transport layer laminated on a substrate are mass-produced commercially because of their advantages such as a high sensitivity and a wide selectivity for the material, which facilitates the preparation of photoreceptors with a high safety, a high productivity of the coating layer, and a relatively low production cost.
On the other hand, rapid progress has been made recently in digitization technology for image formation for obtaining images with higher quality and for enabling memorizing and free editing of input images. Hitherto, a digital image formation has been possible only with certain devices such as laser printers and LED printers, which are output devices of word processors or personal computers, and parts of color laser copiers, but digitization is rapidly prevailing in the field of ordinary copying machines which have mostly been designed for analog image formation.
For carrying out such digital image formation, when a computer information is directly used, an electrical signal of such information is converted to an optical signal, or when information is input from a manuscript, such information is read as optical information. The obtained optical signal is converted to the digital electrical signal, and then the obtained signal is again converted to the optical signal and input to the photoreceptor. In either case, the information is input as optical signal to the photoreceptor, and laser light or LED light is principally used for the optical input of such digital signal. Input light most popularly used at present is near infrared light with an oscillation wavelength of 780 nm or 660 nm, or long-wavelength light with a wavelength close thereto. The primary requirement for a photoreceptor used for digital image formation is that it has enough sensitivity to such near infrared light and long-wavelength light, and a variety of materials have been studied for such photoreceptor. Phthalocyanine compounds have been most earnestly studied, with some of such compounds having already been put to practical use, as many of these compounds are relatively easy to synthesize and have a high sensitivity to long-wavelength light.
For instance, a photoreceptor using titanyl phthalocyanine is disclosed in U.S. Pat. No. 4,725,519, and use of .beta.-indium phthalocyanine is proposed in U.S. Pat. No. 4,471,039. Also, Japanese Patent Application Laid-open (Kokai) No. 2-233769 discloses a photoreceptor using .chi.-type metal-free phthalocyanine, and in U.S. Pat. No. 4,557,868 the use of vanadyl oxyphthalocyanine is proposed as a photoreceptor material.
On the other hand, for digital image formation, there is prevalently employed a so-called reversal developing system in which toner is deposited at the portion exposed to light for the purpose of making effective utilization of light or increasing resolving power. In the reversal developing process, the dark potential portion appears as a white area and the bright potential portion as a black area (image portion).
As mentioned above, the photoreceptor, after image development, is subjected to initialization for image formation. In this step, the charge erasure is conducted either by a method utilizing AC corona discharge or light. The method utilizing light, namely charge erasure by light, is preferred as the apparatus used for this method is simple and no harmful gas such as ozone is produced unlike in the method utilizing an AC corona discharge.
However, in the case where the present inventors conducted the image formation using a laminate-type photoreceptor containing a phthalocyanine compound in the charge generation layer according to a reversal development copying process including the step of charge erasure by light, a phenomenon was observed in which the image formed by the process of the first rotation cycle of the photoreceptor exhibited excessive background fouling and no good image could be obtained. When the copying process was conducted a plural number of times to form plural copies of an image, the second image prepared on the second rotation cycle of the photoreceptor, although showing slight background staining, was nevertheless substantially acceptable, and for each succeeding rotation of the photoreceptor, almost good images were obtained.
According to the investigation of this phenomenon by measuring surface potential of the photoreceptor, it has been found that the surface potential in the development stage of the first rotation cycle is substantially below the prescribed level. In the second rotation cycle, only a slightly diminished surface potential was observed, and in the third and succeeding rotation cycles of the photoreceptor, the surface potential was retained at the prescribed level.
When the same measurement was conducted after allowing the apparatus to stand for a while, a similar phenomenon was observed in the first rotation cycle. Further, when the same measurement was conducted after using the photoreceptor repeatedly to bring it into a considerably fatigued state, it was found that the lowering of the surface potential was further enlarged in the first rotation cycle.
The laminate-type photoreceptors using a phthalocyanine compound in the charge generation layer are widely used, and the above phenomenon has been observed in use of any of these photoreceptors, although there was a slight difference in degree among them. The fact that the above phenomenon has not been observed, in the case of the laminate-type photoreceptors using an azo dye in the charge generation layer, indicates that the above phenomenon is specific to the laminate-type photoreceptors using the phthalocyanine compound.
The mechanism of this phenomenon is not clear, but various investigations point to the following facts for accounting for the above phenomenon.
In the operation for charge erasure by light in an ordinary electrophotographic process, the carrier is formed in excess in the charge generation layer of the laminate-type photoreceptor to neutralize the residual potential, thereby erasing the electrical charge. Here, when electron traps are present in the charge generation layer, the previously formed carrier is temporarily captured by such traps, and if such carrier still remains in the ensuing charging step, a part thereof is released which causes a lowering of the charging potential. In the second and succeeding rotations, since the electron traps are almost plugged up, the release of the carrier in the next charging step is restricted, thereby lessening the lowering of the charging potential.
It is also considered that when allowed to stand, the electron holes captured by the traps are heat-relaxed, thereby allowing the electron traps to be restored to their initial free state which causes a lowering of the potential. Enlargement of potential decrement observed when the photoreceptor is fatigued is considered attributable to gradual increase of the amount of electron traps in the charge generation layer due to fatigue.
As explained above, when a laminate-type photoreceptor using a phthalocyanine compound in the charge generation layer is employed in a reversal developing electrophotographic process including a step for charge erasure by light, the above-mentioned problem exists potentially. In the past, such problem has been countered by employing a system in which the process of the first rotation of photoreceptor, where the charging voltage lowers, is not used for image formation (that is, photoreceptor is rotated idly), and the image formation is conducted in the processes of the second and succeeding rotations where the charging voltage is stabilized. This system has been employed for the reasons that in a reversal development-type printer with a relatively low copying speed (such as less than 10 copies per minute with A4 size paper), the above phenomenon does not occur conspicuously because the charge controlling capacity of the charger has enough and to spare, and that no problem arises even when the first rotation of photoreceptor is made idle since time is required for transfer of data from a computer, etc. However, in case the manuscript is copied directly as in a digital copier with a high copying speed, the incorporation of such idle rotation becomes a great obstacle to high-speed operation, and thus it has been ardently desired to develop a system in which the image formation can be conducted from the first rotation of photoreceptor.
In view of the above, as a result of the present inventors' extensive studies on techniques dispensing with such wasteful idle rotation when a laminate-type photoreceptor using a phthalocyanine compound in the charge generation layer is employed in a reversal developing electrophotographic copying process including a step for charge erasure by light, it has been found that by employing a system in which charge erasure by light is not conducted in the image formation in the process of the first rotation of photoreceptor but is practiced in the second and succeeding rotations, it is possible to obtain high-quality images continuously without making idle the first rotation. The present invention has been achieved on the basis of the above finding.